Abstract: Rainfall-induced landslides are clustery, recorded by Chinese historic documents. Study of early-warning is a significant way to prevent and reduce damages. Conventional forecasting model is based on statistics of landslides, which is short of inducement mechanism and mechanical mechanism, with limited dependability and precision of forecasting. The typical landslide in weathered granitic crust area is studied in this paper. Based on simplified geological model and saturated-unsaturated seepage theory, the process of rainfall infiltration and instability mechanism is studied, then the warning criterion of typical slope is established. (1) Typical dual-structure slopes in weathered granite crust area are soil-like slopes and covering soil is thick. They are divided into two parts on profile. The upper part is collusive clay with loose soil and high permeability, while the lower part is residual clay with compactness soil and lower permeability. (2) Rainfall infiltration under several conditions are simulated. Taking rainfall intensity of 50mm·d^-1 for example, within 30 hours, most rain is in upper collusive clay, causing front edge of slope saturated and sliding zone hydropsy. From 40 to 50 hours, infiltration along with tension cracks around rear edge of slope is faster, and water in cracks may generate hydrostatic increasing weight of soil and sliding power. Meanwhile toe of slope saturates and breaks firstly. Then landslide occurs. (3) Porewater pressure increases continually from monitoring in three parts of slope(front, central and rear of slope). Namely slope soil transforms unsaturated to saturated. The toe of slope gets saturated first, central part under continual infiltration, and upper part saturation cause sliding zone deformation and sliding through hydrostatic pressure. (4) Durations needed when landslides occur under different rainfall conditions are:13 days with moderate rain(10mm·d^-1), 5 days with heavy rain(25mm·d^-1), 2.2 days with rainstorm(50mm·d^-1), and 1.1 days with extraordinary rainstorm(100mm·d^-1). Duration of hysteresis is about 5 hours when rainstorm. Finally, critical rainfall criterion(I-D curve) of this kind of slopes is established.